The invention relates generally to optically amplified lightwave communication systems and, more particularly, to controlling transient response in such systems.
To meet the increasing demands for more bandwidth and higher data rates in today""s networks, wavelength division multiplexing (WDM) is being used extensively in long haul optical transmission systems and is being contemplated for use in short haul applications, such as metropolitan area networks and the like. As is well known, WDM combines many optical channels each at a different wavelength for simultaneous transmission as a composite optical signal in a single optical fiber.
Optical amplifiers are commonly used in lightwave communication systems as in-line amplifiers for boosting signal levels to compensate for losses in a transmission path, as power amplifiers for increasing transmitter power, and as pre-amplifiers for boosting signal levels before receivers. In WDM systems, optical amplifiers are particularly useful because of their ability to amplify many optical channels simultaneously. Rare earth-doped fiber optical amplifiers, e.g., erbium-doped fiber amplifiers, are commonly used in WDM systems, although other types of optical amplifiers, e.g., semiconductor optical amplifiers, can also be used.
In an optically amplified WDM system, signal power transients in a WDM signal can be a significant problem. Signal power transients may occur as a result of adding or dropping individual optical channels, network reconfigurations, failures or recovery from failures, and so on. For example, adding or dropping individual channels of a WDM signal may cause changes in input power to an optical amplifier, which in turn results in changes in gain as well as fluctuations of power levels in surviving optical channels, i.e., those optical channels that are still present in the WDM signal after an add/drop has occurred. Stated otherwise, because optical amplifiers in WDM systems are typically operated in saturation, the output power of an optical amplifier will not necessarily change in a corresponding manner with input power changes and, as a result, optical power in the individual surviving channels will fluctuate undesirably. These power fluctuations may result in unnecessary protection switches in the network, transmission stabilization problems, unacceptable bit error rate degradation if power variations are not within the dynamic range of receiver equipment, and other power-related problems.
Several gain control schemes have been proposed for reducing the effects of power transients. For example, U.S. patent application Ser. No. 09/382853, entitled xe2x80x9cFast Gain Control for Optical Amplifiersxe2x80x9d, which is incorporated by reference herein, describes one approach for reducing the effects of signal power transients in an optically amplified WDM network. In this approach, per-channel gain of individual optical channels is kept relatively constant despite changes in input power at the optical amplifier, such as when individual optical channels of the WDM signal are added/dropped. By maintaining relatively constant per-channel gain in an amplified WDM signal despite changes in input power at the optical amplifier, power fluctuations are substantially reduced in surviving optical channels of the WDM signal.
However, even when a gain control scheme is employed, there still may be problems relating to power transients that may perpetuate in the network depending on network topology and other factors. For example, gain-controlled optical amplifiers may compensate for large power transients, but typically will not achieve complete suppression of low level signal variations. In particular, so-called remnants of the power transients may still perpetuate around the network. As used herein, power transient is meant to correspond to the initial power-affecting change where it is desirable to respond to the transient event, e.g., the aforementioned gain control to respond to a change in channel count. Remnants of power transients, or artifacts as they are sometimes referred to, are typically a result of imperfect approximations that are made when responding to the initial transient event, e.g., approximations of the amount of required gain adjustment. Remnants may be in the form of oscillations of the initial power transient that perpetuate as an error signal around the network. Unless attenuated, these remnants may de-stabilize or otherwise disturb the network. For example, remnants that are continuously routed around a network may trigger unwanted effects if, for example, a subsequent optical amplifier cannot distinguish the remnants from the initial power transient caused by an actual transient event.
Remnants can be especially problematic in particular network topologies, such as a ring network. A WDM ring network, as is well-known, typically includes a plurality of interconnected nodes, at which WDM optical signals may be amplified and at which individual optical channels may be added or dropped. In a WDM ring network, remnants may continue to circulate around the ring getting further amplified as they pass through subsequent nodes. Consequently, the probability of remnants triggering an undesirable response increases in a network topology such as a ring. Moreover, during amplification, well-known cross-saturation effects (e.g., gain at a wavelength is affected by power present at other wavelengths) may imprint the relatively low frequency components of amplitude variations of a signal at one wavelength on signals at other wavelengths. Similarly, during high power operation, non-linear effects in the fiber also may transfer such amplitude variations from one wavelength to another. Thus, even if a channel with remnants is dropped at a node, the effects of the original power transient may still persist in channels that continue to propagate in the ring.
In optically amplified wavelength division multiplexed (WDM) networks, response to power transients is controlled according to the principles of the invention in such a way that control circuitry responds only to power transients caused by an actual transient event and not,to remnants of those power transients that propagate around the network. More specifically, a variable bandwidth filter circuit according to the principles of the invention operates at a first prescribed bandwidth during a first time period xcfx840 to detect a change in signal power (i.e., power transient) caused by a transient event, and operates at a second prescribed bandwidth that is less than the first prescribed bandwidth after the first period of time elapses, e.g., xcfx840+xcex94xcfx84, to filter out low level signal variations, such as remnants of the power transient, noise, and so on. In this way, the power transient related to the actual transient event is preserved to trigger control circuitry, e.g., amplifier gain control based on input power changes, while remnants are filtered out to prevent unwanted responses, e.g., remnants misinterpreted as actual transient events.
According to one illustrative embodiment, the variable bandwidth filter circuit includes a band splitter for splitting an input signal into its low and high frequency signal components. The low frequency signal components are routed in a first transmission path having a low frequency amplifier while the high frequency signal components are routed in a second transmission path having a high frequency amplifier. A switch is employed in the second path to either pass or block the transmission of the high frequency components of the input signal depending on whether a power transient is detected by a transient detector. A band adder combines the signal components from the first and second paths to form an output signal that can then be used for subsequent transient control processing, e.g., optical amplifier gain control. By passing both high and low frequency components of the input signal when a transient event is detected, the bandwidth is effectively xe2x80x9copenedxe2x80x9d to capture both the high and low frequency signal components of the power transient in the input signal. By passing only the low frequency components of the input signal when a power transient is not detected, the bandwidth is effectively xe2x80x9cclosedxe2x80x9d or reduced so that only slow variations in the input signal are passed along.